Effect of heat treatment at 1150 C on creep-rupture properties of alloy FA-180

The alloy FA-180, with a composition of Fe-28Al-5Cr-0.5Nb-0.8Mo-0.025Zr-0.05C-0.005B (at.%), is of interest because of its improved creep-rupture resistance when compared to alloy FA-129 (Fe-28Al-5Cr-0.5Nb-0.2C). At a temperature of 593 C and under a stress of 207 MPa, the creep-rupture life of FA-129 heat treated for 1 h at 750 C is about 20 h while the FA-180 alloy lasts approximately 100 h. Heat treatment at 1,150 C has been shown to further improve the creep life of FA-180 and creep-rupture lives of approximately 2,000 h have been attained. This strengthening was attributed to the presence of fine matrix and grain boundary Zr-rich MC precipitates that were produced by the heat treatment. The current study continues the investigation of the effect of heat treatment at 1,150 C on the improvement of creep-rupture life in alloy FA-180. As part of the effort to understand the strengthening mechanisms involved with heat treatment at 1,150 C, transmission electron microscopy was used to correlate the microstructure with the improved creep resistance. Results indicate that heat treatment at 1,150 C for 1 h, followed by rapid quenching in water or mineral oil, produces even further improvements in the creep-rupture life of this alloy. A specimen being tested at 593 C and 207 MPa was stopped after over 6000 h of life, while another specimen lasted over 1,600 h at 650 C and 241 MPa. The microstructure of the oil-quenched specimen contained many dislocation loops which were not present in the air-cooled specimens. These loops pinned dislocations during creep testing at temperatures of 593--700 C, resulting in stabilized deformation microstructure and increased creep-rupture strength.